Method of Improving Surface Cure in Digital Flexographic Printing Plates

ABSTRACT

A photocurable relief image printing element is described. The photocurable relief image printing element comprises (a) a support layer; (b) one or more photocurable layers disposed on the support layer, wherein the one or more photocurable layers comprise: (i) a binder; (ii) one or more monomers; (iii) a photoinitiator; and (iv) an additive selected from the group consisting of phosphites, phosphines, thioether amine compounds, and combinations of one or more of the foregoing; (c) a laser ablatable masking layer disposed on the one or more photocurable layers, the laser ablatable masking layer comprising a radiation opaque material; and (d) optionally, a removable coversheet. The photocurable relief image printing element provides improved surface cure in digital relief image printing elements.

FIELD OF THE INVENTION

The present invention relates generally to a method of improving surfacecure in digital flexographic printing plates.

BACKGROUND OF THE INVENTION

Flexography is a method of printing that is commonly used forhigh-volume runs. Flexography is employed for printing on a variety ofsubstrates such as paper, paperboard stock, corrugated board, films,foils and laminates. Newspapers and grocery bags are prominent examples.Coarse surfaces and stretch films can be economically printed only bymeans of flexography.

Flexographic printing plates are relief plates with image elementsraised above open areas. Generally, the plate is somewhat soft, andflexible enough to wrap around a printing cylinder, and durable enoughto print over a million copies. Such plates offer a number of advantagesto the printer, based chiefly on their durability and the ease withwhich they can be made. A typical flexographic printing plate asdelivered by its manufacturer is a multilayered article made of, inorder, a backing or support layer; one or more unexposed photocurablelayers; optionally a protective layer or slip film; and often, aprotective cover sheet.

The support (or backing) layer lends support to the plate. The supportlayer can be formed from a transparent or opaque material such as paper,cellulose film, plastic, or metal. Preferred materials include sheetsmade from synthetic polymeric materials such as polyesters, polystyrene,polyolefins, polyamides, and the like. One widely used support layer isa flexible film of polyethylene terephthalate.

The photocurable layer(s) can include any of the known polymers,monomers, initiators, reactive and/or non-reactive diluents, fillers,and dyes. As used herein, the term “photocurable” refers to acomposition which undergoes polymerization, cross-linking, or any othercuring or hardening reaction in response to actinic radiation with theresult that the unexposed portions of the material can be selectivelyseparated and removed from the exposed (cured) portions to form athree-dimensional relief pattern of cured material. Exemplaryphotocurable materials are disclosed in European Patent Application Nos.0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No.1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No.3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos.4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S.Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz,et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No.4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., thesubject matter of each of which is herein incorporated by reference inits entirety. More than one photocurable layer may also be used.

Photocurable materials generally cross-link (cure) and harden throughradical polymerization in at least some actinic wavelength region. Asused herein, “actinic radiation” is radiation that is capable ofpolymerizing, crosslinking or curing the photocurable layer. Actinicradiation includes, for example, amplified (e.g., laser) andnon-amplified light, particularly in the UV and violet wavelengthregions.

The slip film is a thin layer, which protects the photopolymer from dustand increases its ease of handling. In a conventional (“analog”) platemaking process, the slip film is transparent to UV light, and theprinter peels the cover sheet off the printing plate blank, and places anegative on top of the slip film layer. The plate and negative are thensubjected to flood-exposure by UV light through the negative. The areasexposed to the light cure, or harden, and the unexposed areas areremoved (developed) to create the relief image on the printing plate.

In a “digital” or “direct to plate” process, a laser is guided by animage stored in an electronic data file, and is used to create an insitu negative in a digital (i.e., laser ablatable) masking layer, whichis generally a slip film which has been modified to include a radiationopaque material. Portions of the laser ablatable layer are then ablatedby exposing the masking layer to laser radiation at a selectedwavelength and power of the laser. Examples of laser ablatable layersare disclosed, for example, in U.S. Pat. No. 5,925,500 to Yang, et al.,and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter ofeach of which is herein incorporated by reference in its entirety.

Processing steps for forming relief image printing elements typicallyinclude the following:

-   -   1) Image generation, which may be mask ablation for digital        “computer to plate” printing plates or negative production for        conventional analog plates;    -   2) Back exposure to create a floor layer in the photocurable        layer and establish the depth of relief;    -   3) Face exposure through the mask (or negative) to selectively        crosslink and cure portions of the photocurable layer not        covered by the mask, thereby creating the relief image;    -   4) Development to remove unexposed photopolymer by solvent        (including water) or thermal development; and    -   5) If necessary, post exposure and detackification.

Removable coversheets are also preferably provided to protect thephotocurable printing element from damage during transport and handling.Prior to processing the printing elements, the coversheet is removed andthe photosensitive surface is exposed to actinic radiation in animagewise fashion. Upon imagewise exposure to actinic radiation,polymerization, and hence, insolubilization of the photopolymerizablelayer occurs in the exposed areas. Treatment with a suitable developersolvent (or alternatively, thermal development) removes the unexposedareas of the photopolymerizable layer, leaving behind a printing reliefthat can be used for flexographic printing.

As used herein “back exposure” refers to a blanket exposure to actinicradiation of the photopolymerizable layer on the side opposite thatwhich does, or ultimately will, bear the relief. This step is typicallyaccomplished through a transparent support layer and is used to create ashallow layer of photocured material, i.e., the “floor,” on the supportside of the photocurable layer. The purpose of the floor is generally tosensitize the photocurable layer and to establish the depth of relief.

Following the brief back exposure step (i.e., brief as compared to theimagewise exposure step which follows), an imagewise exposure isperformed utilizing a digitally-imaged mask or a photographic negativemask, which is in contact with the photocurable layer and through whichactinic radiation is directed.

The type of radiation used is dependent on the type of photoinitiator inthe photopolymerizable layer. The digitally-imaged mask or photographicnegative prevents the material beneath from being exposed to the actinicradiation and hence those areas covered by the mask do not polymerize,while the areas not covered by the mask are exposed to actinic radiationand polymerize. Any conventional sources of actinic radiation can beused for this exposure step. Examples of suitable visible and UV sourcesinclude carbon arcs, mercury-vapor arcs, fluorescent lamps, electronflash units, electron beam units, LEDs and photographic flood lamps.

After imaging, the photosensitive printing element is developed toremove the unpolymerized portions of the layer of photocurable materialand reveal the crosslinked relief image in the cured photosensitiveprinting element. Typical methods of development include washing withvarious solvents or water, often with a brush. Other possibilities fordevelopment include the use of an air knife or thermal development,which typically uses heat plus a blotting material. The resultingsurface has a relief pattern, which typically comprises a plurality ofdots that reproduces the image to be printed. After the relief image isdeveloped, the resulting relief image printing element may be mounted ona press and printing commenced. In addition, if necessary, after thedevelopment step, the relief image printing element may be post exposedand/or detackified as is generally well known in the art.

The shape of the dots and the depth of the relief, among other factors,affect the quality of the printed image. In addition, it is verydifficult to print small graphic elements such as fine dots, lines andeven text using flexographic printing plates while maintaining openreverse text and shadows. In the lightest areas of the image (commonlyreferred to as highlights) the density of the image is represented bythe total area of dots in a halftone screen representation of acontinuous tone image. For Amplitude Modulated (AM) screening, thisinvolves shrinking a plurality of halftone dots located on a fixedperiodic grid to a very small size, the density of the highlight beingrepresented by the area of the dots. For Frequency Modulated (FM)screening, the size of the halftone dots is generally maintained at somefixed value, and the number of randomly or pseudo-randomly placed dotsrepresent the density of the image. In both cases, it is necessary toprint very small dot sizes to adequately represent the highlight areas.

Maintaining small dots on flexographic plates can be very difficult dueto the nature of the platemaking process. In digital platemakingprocesses that use a UV-opaque mask layer, the combination of the maskand UV exposure produces relief dots that have a generally conicalshape. The smallest of these dots are prone to being removed duringprocessing, which means no ink is transferred to these areas duringprinting (i.e., the dot is not “held” on plate and/or on press).Alternatively, if the dots survive processing they are susceptible todamage on press. For example small dots often fold over and/or partiallybreak off during printing, causing either excess ink or no ink to betransferred.

As described in U.S. Pat. No. 8,158,331 to Recchia and U.S. Pat. Pub.No. 2011/0079158 to Recchia et al., the subject matter of each of whichis herein incorporated by reference in its entirety, it has been foundthat a particular set of geometric characteristics define a flexo dotshape that yields superior printing performance, including but notlimited to (1) planarity of the dot surface; (2) shoulder angle of thedot; (3) depth of relief between the dots; and (4) sharpness of the edgeat the point where the dot top transitions to the dot shoulder.

Furthermore, in order to improve surface cure, it has generally beenfound that it can be beneficial to perform additional procedures and/oruse additional equipment. These additional procedures and/or equipmentmay include:

(1) laminating a membrane onto the surface of the photopolymer;

(2) purging oxygen from the photopolymer using an inert gas; or

(3) imaging the photopolymer with a high intensity UV source.

Purging oxygen from the photopolymer using an inert gas typicallyinvolves placing the photocurable resin plate in an atmosphere of inertgas, such as carbon dioxide gas or nitrogen gas, before exposure, inorder to displace the environmental oxygen. A noted drawback to thismethod is that it is inconvenient and cumbersome and requires a largespace for the apparatus.

Another approach involves subjecting the plates to a preliminaryexposure (i.e., “bump exposure”) of actinic radiation. During bumpexposure, a low intensity “pre-exposure” dose of actinic radiation isused to sensitize the resin before the plate is subjected to the higherintensity main exposure dose of actinic radiation. The bump exposure istypically applied to the entire plate area and is a short, low doseexposure of the plate that reduces the concentration of oxygen, whichinhibits photopolymerization of the plate (or other printing element)and aids in preserving fine features (i.e., highlight dots, fine lines,isolated dots, etc.) on the finished plate. However, thepre-sensitization step can also cause shadow tones to fill in, therebyreducing the tonal range of the halftones in the image. In thealternative, a selective preliminary exposure, as discussed for examplein U.S. Patent Publication No. 2009/0043138 to Roberts et al., thesubject matter of which is herein incorporated by reference in itsentirety, has also been proposed. Other efforts to reduce the effects ofoxygen on the photopolymerization process have involved special plateformulations alone or in combination with the bump exposure.

As is readily apparent, all of these techniques involve unavoidablecosts such as capital expenses on equipment, space, consumables, inertgases, license fees, etc. Thus, it would be desirable to provide arelief image printing element having improved surface cure withoutneeding to subject the printing element to any additional processingsteps. Also, it would be desirable to provide a digital relief imageprinting element that comprises printing dots having a desirable shapeand desirable depth of relief without needing to subject the reliefimage printing element to any additional procedures.

The inventors of the present invention have found that the inclusion ofparticular additives in the photocurable layer of the relief imageprinting element, as described herein, produces relief image printingelement that comprise dots having desirable shapes and depth of relief.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a relief imageprinting element having improved surface cure.

It is another object of the present invention to provide a method oftailoring or modifying the shape of relief printing dots in a reliefimage printing element for optimal printing on various substrates and/orunder various conditions.

It is another object of the present invention to provide an improvedmethod of producing relief image printing elements comprising dotshaving desirable geometric characteristics.

It is still another object of the present invention to greatlystreamline the workflow of the digital platemaking process.

It is another object of the present invention to provide an improvedmethod of creating a relief image printing element having tailoredrelief dots in terms of edge definition, shoulder angle, and/or printsurface.

To that end, in one embodiment, the present invention relates generallyto a photocurable relief image printing element comprising:

-   -   a) a support layer;    -   b) one or more photocurable layers disposed on the support        layer, wherein the one or more photocurable layers comprise:        -   i) a binder;        -   ii) one or more monomers;        -   iii) a photoinitiator; and        -   iv) an additive selected from the group consisting of            phosphites, phosphines, thioether amine compounds, and            combinations of one or more of the foregoing;    -   c) a laser ablatable masking layer disposed on the one or more        photocurable layers, the laser ablatable masking layer        comprising a radiation opaque material; and    -   d) optionally, a removable coversheet.

In another embodiment, the present invention relates generally to amethod of producing a relief image printing element from a photocurableprinting blank, the method comprising the steps of:

-   -   a) providing a photocurable printing blank, the photocurable        printing blank comprising:        -   i) a backing or support layer;        -   ii) one or more photocurable layers disposed on the backing            or support layer, wherein the one or more photocurable            layers comprise:            -   1) a binder;            -   2) one or more monomers;            -   3) a photoinitiator; and            -   4) an additive selected from the group consisting of                phosphites, phosphines, thioether amine compounds, and                combinations of one or more of the foregoing;        -   iii) a laser ablatable masking layer disposed on the at            least one photocurable layer, the laser ablatable masking            layer comprising a radiation opaque material;    -   b) selectively ablating the laser ablatable mask laser to create        an in situ negative of a desired image in the laser ablatable        mask layer;    -   c) exposing the at least one photocurable layer to actinic        radiation through the in situ negative to selectively crosslink        and cure portions of the at least one photocurable layer; and    -   d) developing the exposed at least one photocurable layer of the        photocurable printing blank to reveal the relief image therein,        said relief image comprising a plurality of relief printing        dots.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A and 1B depict SEM pictures of SPF778, SPF779 and SPF771processed in solvent.

FIGS. 2A and 2B depict SEM pictures of SPF778, SPF779 and SPF771processed thermally.

FIG. 3 depicts average dot shoulder angles of SPF778 and SPF779 withrespect to dot-on-file.

FIG. 4 depicts Shore A hardness of various 67 mil plates investigated.

FIG. 5 depicts dot gain curves of thermally processed SPF778 and SPF779.

FIG. 6 depicts actual dot sizes of SPF778, SPF779, SPF771, SPF802 andSPF803 processed in solvent with respect to dot-on-file.

FIGS. 7A and 7B depict SEM pictures (150×) of solvent processed SPF802and SPF803, as a function of various dot-on-file % at 150 lpi.

FIG. 8 depicts surface roughness (SR) of SPF771 and SPF778 upon thermalprocessing with respect to hot roll temperatures.

FIG. 9 depicts SEM pictures (150×) of SPF814 processed A) in solvent andB) thermally.

FIG. 10 depicts a means of characterizing the planarity of a dot'sprinting surface where p is the distance across the dot top, and r_(t)is the radius of curvature across the surface of the dot.

FIG. 11 depicts a flexo dot and its edge, where p is the distance acrossthe dot top. This is used in the characterization of edge sharpnessr_(e):p, where r_(e) is the radius of curvature at the intersection ofthe shoulder and the top of the dot.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Upon UV-curing in the presence of oxygen, the surface properties of agiven photopolymer can be quite different from its bulk properties. Thisis mainly because oxygen inhibition strongly affects the surface layerupon UV exposure, thus disproportionately suppressing the curingreaction in the surface layer compared to the bulk. As a result, themost desirable properties for end-use product performance are notattained. In addition, the poor surface cure can significantly alter thesize and shape of the relief features formed in the photopolymer by UVcuring.

One such example is found in digital flexographic printing plates. Dueto problems with curing the photopolymer layer, the following effectsare typically observed:

-   -   (1) Smaller dot sizes than intended. Digital plates require        “bumps” that artificially employ larger dot sizes in an        electronic file to render the intended sizes on the plates, thus        truncating the overall tonal.    -   (2) Poorly defined imaging edges particularly in the dot shapes,        i.e., round top dots (RTD) instead of flat top dots (FTD), which        contribute to high dot gains upon impression during printing.    -   (3) High surface roughness of solids upon thermal processing,        which adversely influences solid ink density (SID).

These shortcomings negatively affect printing quality and often timeseven limit the applications of digital plates.

To overcome these drawbacks, the inventors of the present invention havefound that the introduction of particular additives into thephotocurable composition of flexographic printing plates greatly improvethe curing reaction in the surface layer. The significance of thisinvention lies in the fact that the surface cure can be greatly beenhanced, without resorting to additional technology (including highintensity UV sources, inert gas chambers, membrane lamination, etc.).Furthermore, the conventional practice of applying bumps to theelectronic files to obtain intended dot sizes can also be reduced oreliminated. In short, the present invention can greatly streamline theplatemaking process and save costs required to support the conventionalprocedure, equipment and techniques, without compromising desirablecharacteristics resulting from good surface cure.

Based thereon, in one embodiment, the present invention relatesgenerally to a photocurable relief image printing element comprising:

-   -   a) a support layer;    -   b) one or more photocurable layers disposed on the support        layer, wherein the one or more photocurable layers comprise:        -   i) a binder;        -   ii) one or more monomers;        -   iii) a photoinitiator; and        -   iv) an additive selected from the group consisting of            phosphites, phosphines, thioether amine compounds, and            combinations of one or more of the foregoing;    -   c) a laser ablatable masking layer disposed on the one or more        photocurable layers, the laser ablatable masking layer        comprising a radiation opaque material; and    -   d) optionally, a removable coversheet.

As described herein, the additives may comprise phosphites, having thegeneral structure P(OR)₃ or P(OAr)₃, phosphines, having the generalstructure PR₃ or PAr₃, thioether amine compounds, or combinations of oneor more of the foregoing. The additive(s) may be used in thephotopolymer composition in an amount of about 0.1 to about 10% byweight, more preferably in an amount of about 0.05 to about 2% byweight.

Suitable phosphites include, but are not limited to,tris(nonylphenyl)phosphate (TNPP) CAS No. 265-78-4, triphenyl phosphite,diphenyl phosphite, tridecyl phosphite, triisodecyl phosphite,tris(tridecyl)phosphite, trilauryl phosphite, disteraryl pentaerythrioldiphosphite, diisodecyl phenyl phosphite, diphenyl isodecyl phosphite,diphenyl octyl phosphite, diphenyl isooctyl phosphite, diphenyl triisodecyl monophenyl dipropyleneglycol diphosphite, alkyl bisphenol Aphosphite, tetraphenyl dipropyleneglycol diphosphite,poly(dipropyleneglycol) phenyl phosphite, tris(dipropylene glycol)phosphite, and dioleyl hydrogen phosphate. In one embodiment, thephosphite comprises TNPP.

Suitable phosphines include, but are not limited to, triphenylphosphine, tri-p-tolylphosphine, diphenylmethylphosphine,diphenylethylphosphine, diphenylpropylphosphine,dimethylphenylphosphine, diethylphenylphosphine,dipropylphenylphosphine, divinylphenylphosphine,divinyl-p-methoxyphenylphosphine, divinyl-p-bromophenylphosphine,divinyl-p-tolylphosphine, diallylphenylphosphine,divinyl-p-bromophenylphosphine, and diallyl-p-tolylphosphine.

Suitable thioether amine compounds include, but are not limited to,2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol(CAS No. 991-84-4),4-[[4,6-bis(nonylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol,4-[[4,6-bis(octadecylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol,4-[[4,6-octylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-bis(2-methylnonan-2-yl)phenol,4-[[4,6-bis(hexylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol,4-[[4,6-bis(heptylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol,4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]amino]-2-tert-butyl-6-methylphenol,4-[[4,6-bis(ethylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol,4-[[4,6-bis(2,4,4-trimethylpentan-2-ylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol,4-[[4,6-bis(2-octylsulfanylethylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol,4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-dibutylphenol,4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-dimethylphenol,2,6-ditert-butyl-4-[[4-(3,5-ditert-butyl-4-hydroxyanilino)-6-octylsulfanyl-1,3,5-triazin-2-yl]amino]phenol,4-[[4,6-bis(pentylsulfanyl)-1,3,5-triazin-2-yl]amino-2,6-dimethylphenol,4-[[4,6-bis(hexylsulfanyl)-1,3,5-triazin-2-yl]amino]-2-tert-butylphenol,2,6-ditert-butyl-4-[(4-octylsulfanyl-1,3,5-triazin-2-yl)amino]phenol,4-[[4,6-bis(ethylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-dimethylphenol,4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]-butylamino]-2,6-ditert-butylphenol,4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]-cyclohexylamino]-2,6-ditert-butylphenol,2-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]amino]-6-tert-butylphenol,2-tert-butyl-6-methyl-4-[[4-octylsulfanyl-6-[(2,2,6,6,-tetramethylpiperidin-4-yl)amino]-1,3,5-triazin-2-yl]amino]phenol,4-[[4,6-bis(octylsulfanylmethyl)-1,3,5-triazin-2-yl)amino]-2,6-ditert-butylphenol,4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl)methylamino]-2,6-ditert-butylphenol,4-[(4-amino-6-chloro-1,3,5-triazin-2-yl)amino]-2,6-ditert-butylphenol,and4-[(4-cyclohexyl-6-cyclohexylsulfanyl-1,3,5-triazin-2-yl)amino]-2,6-di(propan-2-yl)phenol.In one embodiment, the thioether amine compound comprises2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol(also referred to asphenol,4-[[4,6-bis(octylthio)-1,3,5-triazin-2-yl]amino]-2,6-bis(1,1-dimethylethyl).

In addition, one or more antioxidants such as1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene,butylated hydroxytoluene (BHT), alkylated phenols, e.g.,2-6-di-tert-butyl-4-methylphenol; alkylated bis-phenols, e.g.,2,2-methylene-bis-(4-methyl-6-tert-butylphenol);2-(4-hydroxy-3,5-di-tert-butylanilino)-4,6-bis-(n-octylthio)-1,3,5-triazine; polymerized trimethyldihydroquinone; and dilaurylthiopropionate can also be used in the compositions of the invention incombination with the above referenced additives to further tailor dotshapes in terms of dot angle, dot tops, etc. In one preferredembodiment, the antioxidant is1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-butyl-4-hydroxybenzyl) benzene,available from Albemarle under the tradename Ethanox 330.

The photopolymerizable composition of the present invention comprisesone or more binders, monomers and plasticizers in combination with oneor more photoinitiators and the above-described additives.

The binder type is not critical to the photopolymer composition andmost, if not all, styrenic copolymer rubbers are usable in thecompositions of the invention. Suitable binders can include natural orsynthetic polymers of conjugated diolefin hydrocarbons, including1,2-polybutadiene, 1,4-polybutadiene, butadiene/acrylonitrile,butadiene/styrene, thermoplastic-elastomeric block copolymers e.g.,styrene-butadiene-styrene block copolymer, styrene-isoprene-styreneblock copolymer, etc., and copolymers of the binders. It is generallypreferred that the binder be present in at least an amount of 60% byweight of the photosensitive layer. The term binder, as used herein,also encompasses core shell microgels or blends of microgels andpre-formed macromolecular polymers.

Non-limiting examples of binders that are usable in the compositions ofthe instant invention include styrene isoprene styrene (SIS), acommercial product of which is available from Kraton Polymers, LLC underthe tradename Kraton® D1161; styrene isoprene butadiene styrene (SIBS),a commercial product of which is available from Kraton Polymers, LLCunder the tradename Kraton® D1171; and styrene butadiene styrene (SBS),a commercial product of which is available from Kraton Polymers LLCunder the tradename Kraton® DX405.

Monomers suitable for use in the present invention areaddition-polymerizable ethylenically unsaturated compounds. Thephotocurable composition may contain a single monomer or a mixture ofmonomers which form compatible mixtures with the binder(s) to produceclear (i.e., non-cloudy) photosensitive layers. The monomers aretypically reactive monomers especially acrylates and methacrylates. Suchreactive monomers include, but are not limited to, trimethylolpropanetriacrylate, hexanediol diacrylate, 1,3-butylene glycol diacrylate,diethylene glycol diacrylate, 1,6-hexanediol diacrylate, neopentylglycol diacrylate, polyethylene glycol-200 diacrylate, tetraethyleneglycol diacrylate, triethylene glycol diacrylate, pentaerythritoltetraacrylate, tripropylene glycol diacrylate, ethoxylated bisphenol-Adiacrylate, trimethylolpropane triacrylate, di-imethylolpropanetetraacrylate, triacrylate of tris(hydroxyethyl) isocyanurate,dipentaerythritol hydroxypentaacrylate, pentaerythritol triacrylate,ethoxylated trimethylolpropane triacrylate, triethylene glycoldimethacrylate, ethylene glycol dimethacrylate, tetraethylene glycoldimethacrylate, polyethylene glycol-200 dimethacrylate, 1,6-hexanedioldimethacrylate, neopentyl glycol dimethacrylate, polyethylene glycol-600dimethacrylate, 1,3-butylene glycol dimethacrylate, ethoxylatedbisphenol-A dimethacrylate, trimethylolpropane trimethacrylate,diethylene glycol dimethacrylate, 1,4-butanediol diacrylate, diethyleneglycol dimethacrylate, pentaerythritol tetramethacrylate, glycerindimethacrylate, trimethylolpropane dimethacrylate, pentaerythritoltrimethacrylate, pentaerythritol dimethacrylate, pentaerythritoldiacrylate, urethanemethacrylate or acrylate oligomers and the likewhich can be added to the photopolymerizable composition to modify thecured product. Monoacrylates including, for example, cyclohexylacrylate, isobornyl acrylate, lauryl acrylate and tetrahydrofurfurylacrylate and the corresponding methacrylates are also usable in thepractice of the invention. Especially preferred acrylate monomersinclude hexanediol diacrylate (HDDA) and trimethylolpropane triacrylate(TMPTA). Especially preferred methacrylate monomers include hexanedioldimethacrylate (HDDMA) and triemethylolpropane trimethacrylate (TMPTA).It is generally preferred that the one or more monomers be present in atleast an amount of 5% by weight of the photosensitive layer.

The photopolymer layer preferably contains a compatible plasticizer,which serves to lower the glass transition temperature of the binder andfacilitate selective development. Suitable plasticizers include, but arenot limited to, dialkyl phthalates, alkyl phosphates, polyethyleneglycol, polyethylene glycol esters, polyethylene glycol ethers,polybutadiene, polybutadiene styrene copolymers, hydrogenated, heavynaphthenic oils, hydrogenated, heavy paraffinic oils, and polyisoprenes.Other useful plasticizers include oleic acid, lauric acid, etc. Theplasticizer is generally present in an amount of at least 10% by weight,based on weight of total solids of the photopolymer composition.Commercially available plasticizers for use in compositions of theinvention include 1,2-polybutadiene, available from Nippon Soda Co.under the tradename Nisso PB B-1000; Ricon 183, which is a polybutadienestyrene copolymer, available from Cray Valley; Nyflex 222B, which is ahydrogenated heavy naphthenic oil, available from Nynas AB; ParaLux2401, which is a hydrogenated heavy paraffinic oil, available fromChevron U.S.A., Inc.; and Isolene 40-S, which is a polyisopreneavailable from Royal Elastomers.

Photoinitiators for the photocurable composition include the benzoinalkyl ethers, such as benzoin methyl ether, benzoin ethyl ether, benzoinisopropyl ether and benzoin isobutyl ether. Another class ofphotoinitiators are the dialkoxyacetophenones such as2,2-dimethoxy-2-phenylacetophenone and2,2-diethoxy-2-phenylacetophenone. Still another class ofphotoinitiators are the aldehyde and ketone carbonyl compounds having atleast one aromatic nucleus attached directly to the carboxyl group.These photoinitiators include, but are not limited to, benzophenone,acetophenone, o-methoxybenzophenone, acenaphthenequinone, methyl ethylketone, valerophenone, hexanophenone, alpha-phenylbutyrophenone,p-morpholinopropiophenone, dibenzosuberone, 4-morpholinobenzophenone,4′-morpholinodeoxybenzoin, p-diacetylbenzene, 4-aminobenzophenone,4′-methoxyacetophenone, benzaldehyde, alpha-tetralone,9-acetylphenanthrene, 2-acetylphenanthrene, 10-thioxanthenone,3-acetylphenanthrene, 3-acetylindone, 9-fluorenone, 1-indanone,1,3,5-triacetylbenzene, thioxanthen-9-one, xanthene-9-one,7-H-benz[de]-anthracene-7-one, 1-naphthaldehyde,4,4′-bis(dimethylamino)-benzophenone, fluorene-9-one, 1′-acetonaphthone,2′-acetonaphthone, 2,3-butanedione, acetonaphthene, benz[a]anthracene7.12 dione, etc. Phosphines such as triphenylphosphine andtri-o-tolylphosphine are also operable herein as photoinitiators.

Preferred photoinitiators for use in the photopolymer compositions ofthe invention include benzyl dimethyl ketal, a commercial product ofwhich is available from BASF under the Tradename Irgacure 651;α-hydroxyketone, a commercial product of which is available from BASFunder the tradename Irgacure 184; and acyl phosphine, a commercialproduct of which is available from Ciba Specialty Chemicals under thetradename Darocur TPO. In one embodiment, it was determined thatIrgacure 651 was the most effective photoinitiator for UV light with thewavelength of ˜365 nm to obtain the benefits described herein, but otherphotoinitiators can also be used, alone or in combination with Irgacure651.

Various dyes and/or colorants may also optionally be used in thepractice of the invention although the inclusion of a dye and/orcolorant is not necessary to attain the benefits of the presentinvention. Suitable colorants are designated “window dyes” which do notabsorb actinic radiation in the region of the spectrum that theinitiator present in the composition is activatable. The colorantsinclude, for example, CI 109 Red dye, Methylene Violet (CI Basic Violet5), “Luxol” Fast Blue MBSN (CI Solvent Blue 38), “Pontacyl” Wool Blue BL(CI Acid Blue 59 or CI 50315), “Pontacyl” Wool Blue GL (CI Acid Blue 102or CI 50320), Victoria Pure Blue BO (CI Basic Blue 7 or CI 42595),Rhodamine 3 GO (CI Basic Red 4), Rhodamine 6 GDN (CI Basic Red 1 or CI45160), 1,1′-diethyl-2,2′-cyanine iodide, Fuchsine dye (CI 42510),Calcocid Green S (CI 44090) and Anthraquinone Blue 2 GA (CI Acid Blue58), etc. The dyes and/or colorants must not interfere with theimagewise exposure.

Other additives including antiozonants, fillers or reinforcing agents,thermal polymerization inhibitors, UV absorbers, etc. may also beincluded in the photopolymerizable composition, depending on the finalproperties desired. Such additives are generally well known in the art.

Suitable fillers and/or reinforcing agents include immiscible, polymericor nonpolymeric organic or inorganic fillers or reinforcing agents whichare essentially transparent at the wavelengths used for exposure of thephotopolymer material and which do not scatter actinic radiation, e.g.,polystyrene, the organophilic silicas, bentonites, silica, powderedglass, colloidal carbon, as well as various types of dyes and pigments.Such materials are used in amounts varying with the desired propertiesof the elastomeric compositions. The fillers are useful in improving thestrength of the elastomeric layer, reducing tack and, in addition, ascoloring agents.

Thermal polymerization inhibitors include, for example, p-methoxyphenol,hydroquinone, and alkyl and aryl-substituted hydroquinones and quinones,tert-butyl catechol, pyrogallol, copper resinate, naphthalamines,beta-naphthol, cuprous chloride, 2,6-di-tert-butyl-p-cresol, butylatedhydroxytoluene (BHT), oxalic acid, phenothiazine, pyridine, nitrobenzeneand dinitrobenzene, p-toluquinone and chloranil. Other similarpolymerization inhibitors would also be usable in the practice of theinvention.

In another embodiment, the present invention relates generally to amethod of producing a relief image printing element from a photocurableprinting blank, the method comprising the steps of:

-   -   a) providing a photocurable printing blank, the photocurable        printing blank comprising:        -   i) a backing or support layer;        -   ii) one or more photocurable layers disposed on the backing            or support layer, wherein the one or more photocurable            layers comprise:            -   1) a binder;            -   2) one or more monomers;            -   3) a photoinitiator; and            -   4) an additive selected from the group consisting of                phosphites, phosphines, thioether amine compounds, and                combinations of one or more of the foregoing;        -   iii) a laser ablatable masking layer disposed on the at            least one photocurable layer, the laser ablatable masking            layer comprising a radiation opaque material;    -   b) selectively ablating the laser ablatable mask laser to create        an in situ negative of a desired image in the laser ablatable        mask layer;    -   c) exposing the at least one photocurable layer to actinic        radiation through the in situ negative to selectively crosslink        and cure portions of the at least one photocurable layer; and    -   d) developing the exposed at least one photocurable layer of the        photocurable printing blank to reveal the relief image therein,        said relief image comprising a plurality of relief printing        dots.

Table 1 summarizes various examples of various formulae of photocurablecompositions prepared in accordance with the present invention. SPF778and SPF779 contain both TNPP and2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenolin combination with Ethanox® 330. SPF802 incorporates2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenoland SPF803 and SPF814 employ TNPP, respectively.

TABLE 1 Various formulae prepared in accordance with the presentinvention (% by wt.) SPF Component 771 SPF778 SPF779 SPF802 SPF803SPF814 Binder 66.38 66.90 66.90 66.93 66.93 66.38 Plasticizer 19.8819.88 19.88 19.88 19.88 19.86 Acrylate 6.59 6.59 6.59 6.59 6.59 6.95Monomer Methacrylate 2.26 2.26 2.26 2.26 2.26 — Monomer Irgacure 6512.95 2.95 2.95 2.95 2.95 5.00 Additive¹ — 0.47 0.25 1.38 — — Ethanox 330— 0.36 0.56 — — — TNPP — 0.55 0.57 — 1.38 1.80 Polymerization 1.92 — — —— — inhibitor Savinyl Red 0.01 0.01 0.01 0.01 0.01 0.01 3BLS Tinuvin1130 0.025 0.025 0.025 — — — Total 100 100 100 100 100 100¹2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol

These various photocurable compositions were imagewise exposed toactinic radiation and developed to reveal the relief image thereincomprising a plurality of relief printing dots. The resulting photocuredcompositions comprising the plurality of relief printing dots were thenexamined. As described herein, the photocurable compositions weredeveloped using a solvent to dissolve away the uncured and uncrosslinkedportions of the photocurable composition or were developed using thermaldevelopment in which the uncured and uncrosslinked portions weresoftened and/or melted and then blotted away. Other means of developingthe photocurable compositions would also be known to those skilled inthe art.

The photocurable relief image printing element preferably has a Shore Ahardness of between about 45 and about 70, more preferably between about50 and about 65.

The improved surface cure in the digital flexographic printing platescan most explicitly be revealed by the dot shapes.

FIGS. 1A, 1B, 2A, and 2B depict SEM pictures of SPF778, SPF779, andSPF771 processed in solvent and thermally, respectively, with regard todot sizes in the linear electronic file used or imaging at 150 lpi. Itis clearly shown that, unlike the conventional RTD of SPF771, FTD arecreated in both SPF778 and SPF779.

For SPF778 and SPF779 the average dot shoulder angles are very small asexhibited in FIGS. 1A, 1B, 2A, and 2B. In one embodiment, an averageshoulder angle, as measured as the angle between a shoulder of the dotacross the width of the dot's top or printing surface is less than about30°, preferably less than about 20°.

In summary, the unique dot shapes of SPF778 and SPF779 are consideredideal for printing because:

-   -   (i) The dots tops are flat and hence require less impression        than conventional RTD, which in turn increases plate longevity        due to less susceptibility to wear.    -   (ii) The pillars of the dots sitting on the bases are        practically vertical (no shoulders), which minimizes potential        dot gains upon impression.    -   (iii) The dot bases are broad, as clearly illustrated in FIGS.        1A, 1B, 2A, and 2B, and thus provide the mechanical stability        necessary to endure impact from the unidirectional dynamic        impression applied during printing.

In addition to FTD without shoulder and the elimination of bumps,surface roughness (SR) of solids upon thermal processing is alsopositively influenced by the improved surface cure, in a manner that thehigher the surface cure, the lower the SR. FIG. 8 displays the SR ofSPF778 and SPF771 upon thermal processing with respect to hot rolltemperature. Over the entire temperature range examined, the SR is lowerfor SPF778 than for SPF771. This is directly translated into a moreforgiving thermal processing. In addition, the lower SR is conducive toincreasing solid ink density (SID) upon printing. Thus, it can be seenthat the present invention benefits both thermal processing and end-useplate performance in the following manner, as a result of the reducedSR:

(a) Wider thermal processing windows; and

(b) High SID.

Finally, the planarity of the top of a dot can be measured as the radiusof curvature across the top surface of the dot, r_(t), as shown in FIG.10. It is noted that a rounded dot surface is not ideal from a printingperspective because the size of the contact patch between the printsurface and the dot varies exponentially with impression force.Therefore, the top of the dot preferably has a planarity where theradius of curvature of the dot top is greater than the thickness of thephotopolymer layer, more preferably twice the thickness of thephotopolymer layer, and most preferably more than three times the totalthickness of the photopolymer layer.

Edge sharpness relates to the presence of a well-defined boundarybetween the planar dot top and the shoulder and it is generallypreferred that the dot edges be sharp and defined, as shown in FIG. 11.These well-defined dot edges better separate the “printing” portion fromthe “support” portion of the dot, allowing for a more consistent contactarea between the dot and the substrate during printing.

Edge sharpness can be defined as the ratio of r_(e), the radius ofcurvature (at the intersection of the shoulder and the top of the dot)to p, the width of the dot's top or printing surface, as shown in FIG.3. For a truly round-tipped dot, it is difficult to define the exactprinting surface because there is not really an edge in the commonlyunderstood sense, and the ratio of r_(e):p can approach 50%. Incontrast, a sharp-edged dot would have a very small value of r_(e), andr_(e):p would approach zero. In practice, an r_(e):p of less than 5% ispreferred, with an r_(e):p of less than 2% being most preferred. FIG. 11depicts a flexo dot and its edge, where p is the distance across the dottop and demonstrates the characterization of edge sharpness, r_(e):p,where r_(e) is the radius of curvature at the intersection of theshoulder and the top of the dot.

1. A photocurable relief image printing element comprising: a) a supportlayer; b) one or more photocurable layers disposed on the support layer,wherein the one or more photocurable layers comprise: i) a binder; ii)one or more monomers; iii) a photoinitiator; and iv) an additiveselected from the group consisting of phosphites, phosphines, thioetheramine compounds, and combinations of one or more of the foregoing; c) alaser ablatable masking layer disposed on the one or more photocurablelayers, the laser ablatable masking layer comprising a radiation opaquematerial; and d) optionally, a removable coversheet.
 2. The photocurablerelief image printing element according to claim 1, wherein the additiveis a phosphite selected from the group consisting oftris(nonylphenyl)phosphate, triphenyl phosphite, diphenyl phosphite,tridecyl phosphite, triisodecyl phosphite, tris(tridecyl)phosphite,trilauryl phosphite, disteraryl pentaerythriol diphosphite, diisodecylphenyl phosphite, diphenyl isodecyl phosphite, diphenyl octyl phosphite,diphenyl isooctyl phosphite, diphenyl tri isodecyl monophenyldipropyleneglycol diphosphite, alkyl bisphenol A phosphite, tetraphenyldipropyleneglycol diphosphite, poly(dipropyleneglycol) phenyl phosphite,tris(dipropylene glycol) phosphite, and dioleyl hydrogen phosphite andcombinations of one or more of the foregoing.
 3. The photocurable reliefimage printing element according to claim 2, wherein the additivecomprises tris(nonylphenyl)phosphate.
 4. The photocurable relief imageprinting element according to claim 1, wherein the additive is aphosphine selected from the group consisting of triphenyl phosphine,tri-p-tolylphosphine, diphenylmethylphosphine, diphenylethylphosphine,diphenylpropylphosphine, dimethylphenylphosphine,diethylphenylphosphine, dipropylphenylphosphine, divinylphenylphosphine,divinyl-p-methoxyphenylphosphine, divinyl-p-bromophenylphosphine,divinyl-p-tolylphosphine, diallylphenylphosphine,divinyl-p-bromophenylphosphine, and diallyl-p-tolylphosphine andcombinations of one or more of the foregoing.
 5. The photocurable reliefimage printing element according to claim 1, wherein the additive is athioether amine compound selected from the group consisting of2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol,4-[[4,6-bis(nonylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol,4-[[4,6-bis(octadecylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol,4-[[4,6-octylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-bis(2-methylnonan-2-yl)phenol,4-[[4,6-bis(hexylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol,4-[[4,6-bis(heptylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol,4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]amino]-2-tert-butyl-6-methylphenol,4-[[4,6-bis(ethylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol,4-[[4,6-bis(2,4,4-trimethylpentan-2-ylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol,4-[[4,6-bis(2-octylsulfanylethylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-ditert-butylphenol,4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-dibutylphenol,4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-dimethylphenol,2,6-ditert-butyl-4-[[4-(3,5-ditert-butyl-4-hydroxyanilino)-6-octylsulfanyl-1,3,5-triazin-2-yl]amino]phenol,4-[[4,6-bis(pentylsulfanyl)-1,3,5-triazin-2-yl]amino-2,6-dimethylphenol,4-[[4,6-bis(hexylsulfanyl)-1,3,5-triazin-2-yl]amino]-2-tert-butylphenol,2,6-ditert-butyl-4-[(4-octylsulfanyl-1,3,5-triazin-2-yl)amino]phenol,4-[[4,6-bis(ethylsulfanyl)-1,3,5-triazin-2-yl]amino]-2,6-dimethylphenol,4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]-butylamino]-2,6-ditert-butylphenol,4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]-cyclohexylamino]-2,6-ditert-butylphenol,2-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl]amino]-6-tert-butylphenol,2-tert-butyl-6-methyl-4-[[4-octylsulfanyl-6-[(2,2,6,6,-tetramethylpiperidin-4-yl)amino]-1,3,5-triazin-2-yl]amino]phenol,4-[[4,6-bis(octylsulfanylmethyl)-1,3,5-triazin-2-yl)amino]-2,6-ditert-butylphenol,4-[[4,6-bis(octylsulfanyl)-1,3,5-triazin-2-yl)methylamino]-2,6-ditert-butylphenol,4-[(4-amino-6-chloro-1,3,5-triazin-2-yl)amino]-2,6-ditert-butylphenol,and4-[(4-cyclohexyl-6-cyclohexylsulfanyl-1,3,5-triazin-2-yl)amino]-2,6-di(propan-2-yl)phenol.In one embodiment, the thioether amino compound comprises2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol(also referred to asphenol,4-[[4,6-bis(octylthio)-1,3,5-triazin-2-yl]amino]-2,6-bis(1,1-dimethylethyl)and combinations of one or more of the foregoing.
 6. The photocurablerelief image printing element according to claim 5, wherein the additivecomprises2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol.7. The photocurable relief image printing element according to claim 1,comprising an antioxidant selected from the group consisting of1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene,butylated hydroxytoluene, alkylated phenols, alkylated bis-phenols,polymerized trimethyldihydroquinone, dilauryl thiopropionate, andcombinations of one or more of the foregoing.
 8. The photocurable reliefimage printing element according to claim 7, wherein the antioxidantcomprises 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene.
 9. The photocurable relief image printing element according toclaim 1, wherein the photocurable composition further comprises one ormore materials selected from the group consisting of plasticizers,antiozonants, fillers, reinforcing agents, thermal polymerizationinhibitors, UV absorbers and combinations of one or more of theforegoing.
 10. The photocurable relief image printing element accordingto claim 9, wherein the one or more materials comprise a plasticizer.11. The photocurable relief image printing element according to claim 1,wherein the photocurable relief image printing element has a Shore Ahardness of between about 45 and about
 70. 12. The photographic reliefimage printing element according to claim 11, wherein the photocurablerelief image printing element has a Shore A hardness of between about 50and about
 65. 13-31. (canceled)